mmdevapi/tests: Remove an unused assignment (PVS-Studio)

[Marvin]

Hi,

While running your changed tests on Windows, I think I found new failures.
Being a bot and all I'm not very good at pattern recognition, so I might be
wrong, but could you please double-check?
Full results can be found at
https://testbot.winehq.org/JobDetails.pl?Key=11941

Your paranoid android.


=== w7u (32 bit render) ===
render.c:1975: Test failed: Got wrong volume: 0.501088
render.c:2004: Test failed: Got wrong volume: 0.501088

=== w8 (32 bit render) ===
render.c:1975: Test failed: Got wrong volume: 0.671337
render.c:2004: Test failed: Got wrong volume: 0.671337